The invention concerns apparatus and methods for the radio transmission of chronometric information.
In order to transmit chronometric pips by radio, use is normally made of a carrier wave provided with a repetitive pattern forming a time marker, equivalent to a brief pulse. The time marker is for example a code of the pseudo-random or “pseudo-noise” type.
It is advantageous to apply this technique to the synchronization of very precise clocks, one being situated for example on the ground and the other on board a satellite. It is a case of precisely measuring the time difference between the respective transitions of these atomic clocks. More generally, it is a case of seeking the time difference between two contemporary events occurring in stations which are distant in space, the word “station” not implying a stationary position here.
One of the stations transmits to the other a time marker related to its own event. Subject to knowing the actual transit time of the waves, the difference measured at the other station between the time marker received and the local event (or a time marker linked to it) gives access to the required time difference. In addition, it can be arranged so that the carrier is frequency linked to the time marker where the phase carrier improves precision.
In order to know the distance travelled by the radio waves, a second transmission is needed in both directions. In the majority of cases, this second transmission must take place at the same time as the first. It will therefore be possible to have two transmissions and one reception for one of the stations, but the other station is the origin of one transmission and two receptions.
In addition, the transmission medium is rarely perfect, and often dispersive. In particular, in the case of an earth-satellite transmission, strong interference is observed when passing through the ionosphere and troposphere. This interference is variable in the short term, and dispersive, with the ionosphere.
The conventional solution consists of using different carrier frequencies for the two transmissions. However, this poses problems, notably with regard to the allocation of frequencies.
There therefore remains only the possibility of effecting the two transmissions with the same carrier frequency. However, this implies shared time, always tricky because of the risk of interference. In addition, since the transit time of the waves between the two stations is greater than the period of recurrence of the time marker, it will happen that the received signal coincides with a moment of transmission, and will therefore be lost by masking, which is unacceptable.
There therefore exists currently no truly satisfactory solution for the radio exchange of chronometric signals between two distant stations.